1. FIELD OF THE INVENTION
The present invention relates to an exhaust gas purifying apparatus for internal combustion engines. In more particular, the invention concerns an exhaust gas purifying apparatus of the type including three-way catalyst a secondary air supply system and an air-fuel ratio detector, wherein the air-fuel ratio detected from the exhaust gas is so controlled that optimum purification condition is provided for the purifying action of the three-way catalyst.
2. DESCRIPTION OF THE PRIOR ART
In general, the so-called three-way catalyst which is supported in a single catalyst floor and serves to oxidize carbon monooxide (CO) and hydrocarbon (HC), while reducing nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine thereby to eliminate these toxic compounds, has purification performance characteristics variable as functions of the air-fuel ratio such as illustrated in FIG. 1 of the accompanying drawings. As can be seen from FIG. 1, the air-fuel ratio detected from the exhaust gas has to be maintained at a value lying in the range shown as hatched in this figure, in order to have the three-way catalyst to be operated at a high purification efficiency.
In the case of hitherto known exhaust gas purifying apparatus, a secondary air supply is made to the exhaust gas conduit upstream of the three-way catalyst in combination with the use of an air-fuel ratio detector for detecting oxygen concentration of the exhaust gas which undergoes variations in dependence on the operating conditions of the engine, whereby the quantity of the secondary air supply is corrected in consideration of the output conditions of the air-fuel ratio detector thereby to assure the optimum air-fuel ratio for the effective action of the three-way catalyst.
As the means for supplying the secondary air, there has been usually employed an air pump which is driven by the engine. Accordingly, the quantity of air discharged from the air pump depends only on the revolution number of the engine independently from the load thereof. In this connection, FIG. 2 of the accompanying drawings shows relation between the air-fuel ratio of the exhaust gas and the suction pressure in the intake conduit or passage of an internal combustion engine under the conditions that a conventional type of the air pump is used with the revolution number of the engine assumed to be constant. On the above assumption, the quantity of the secondary air supply remains to be constant because of the constant revolution number of the engine. In contrast thereto, the quantity of the exhaust gas will vary in dependence on the load of the engine (i.e. the exhaust gas increases as the load becomes greater). For these reasons, in a conventional secondary air supply system in which the quantity of the secondary air supply is preadjusted to be optimum under its maximum load operation of the engine, the air-fuel ratio of the exhaust gas becomes thinner as the load is lower, as shown in FIG. 2.
In general, in the case where the secondary air supply from the air pump is controlled by means of the air-fuel ratio detector, the control is usually effected in an intermittent manner (i.e. on-off control) in consideration of the response characteristics of the control system. In such control system, a control valve apparatus is provided in the secondary air supply conduit to control the secondary air flow and the air-fuel ratio of the exhaust gas having been mixed with the secondary air is detected by the air-fuel ratio detector. When the output signal voltage from the detector is higher than a preset reference voltage, then the air control valve is opened fully. On the other hand, when the detector output is lower than the reference voltage, the air valve is closed completely. In the fully opened position of the air valve, all the secondary air quantity supplied from the air pump is relieved and in the fully closed position of the air valve, all the secondary air quantity is fed to the exhaust conduit without relief, while in the intermediate valve position between the fully opened and the completely closed positions, appropriate relief of the secondary air depends on the valve position and/or the pressure of the secondary air. In this manner, the air-fuel ratio of the exhaust gas is maintained at values falling within the hatched range shown in FIG. 1 thereby to attempt to attain an effective elimination of the toxic components through the action of the three-way catalyst.
In connection with the control system described above, it will be noted that when the secondary air control valve is completly closed, the air-fuel ratio set by the carburetor is effective, while the air-fuel ratio of the exhaust gas exhibiting such behavior as shown in FIG. 2 becomes effective when the secondary air valve is fully opened.
Referring to FIG. 3 of the accompanying drawings, it is known that more than 80% of purification ratio can be attained in respect to all the compounds CO, HC and NOx by using the three-way catalyst in the air-fuel ratio region indicated by the hatched lines. In this figure, discrete points labelled with associated negative suction pressures in the intake conduit of the engine represent values of the air-fuel ratio of the exhaust rewriten from the graph of FIG. 2. As can be seen from FIG. 3, the air-fuel ratio of the exhaust gas is more remarkably deviated from the range which allows more than 80% of purification of the toxic compounds described above, as the load of the engine becomes reduced. In other words, it is impossible to attain satisfactory purification of the exhaust gas in the low load range with the hitherto known control system where the secondary air supply from the air pump driven by the engine is controlled by the air-fuel ratio detector disposed in the exhaust conduit of the engine.